This time I focused on trying to fix the Alpha calculation, trying to get a more correct Alpha values for all the volume pixels. Apparently this has fixed the problems with Alpha > 1.0 so I could remove the check I used in my previous fix.

Rendering over transparent background (this is the problem that was not fixed with my previous fix and it's hopefully solved now):

Before this fix, incorrect:

After this fix, apparently correct:

Example of composition made in GIMP with a background image and the new (correct) volume generated with transparent background:

Maybe it's not yet a perfect solution (in the composition I can still see that the edge of the volume is not 100% transparent, but I still believe it's working much better than before. To make it perfect I would need help from the experts and senior developers as I don't yet understand well the internals of the renderer.

In any case, I hope you find this useful. However, as it's a big change in a main component of YafaRay, please test it thoroughly with your scenes to make sure it does not cause any new issues.